A process control system implements a controller to shape the response of a process to an input signal. The control system can add gain, time varying properties, frequency components, or a combination of these characteristics to the process signal. By properly choosing these characteristics, the control system can stabilize the response of the process, determine overshoot, set acceptable error bounds and satisfy other performance criteria.
A two degree of freedom controller is generally implemented in a two degree of freedom system. Such a two degree of freedom system could consist of a setpoint and a disturbance. Within this system, the controller should track the setpoint and reject any disturbances. Controllers of this type, for example, include the precompensator 10 of FIG. 1.
The precompensator 10 of FIG. 1 includes a prefilter 12 and a load controller 14. These two control elements 12 and 14 shape a process input 16 for a process 18. The prefilter 12 shapes a prefilter response 20 to an input variable 22. The load controller 14 shapes the input 16 to the process 18 based on the prefilter response 20 and a process state 30 that is feedback for the system. The process state 30 is altered by a second variable 32 and the transfer function 36 of the second variable 32.
In the configuration of the precompensator 10, the load controller 14 must shape the process input 16 based in part on the prefilter response 20. Any inaccuracies from error in the prefilter 12 are propagated through the load controller 14.